The present invention is directed to a sensor and system for measuring movement between two points. More particularly, the sensor and system are useful for measuring movement between two points over a large range, and more specifically has use for the movement of roofs and floors in subterranean excavations such as in coal mines.
The collapse of a roof in a coal mine (roof fall) is a costly event in terms of mine safety, mine productivity and the financial impact on mining operations.
Continued mining operations after a roof fall require manual cleanup of the rock and debris. This labor intensive process can last for several days, during which time mining operations must be suspended.
A typical mine usually has a great number of potential roof fall areas, and it has been found that unstable roofs are characterized by: (1) a total convergence between the roof and the floor of the mine greater than one inch following mining operations; (2) convergent velocities between the roof and floor of the mine greater than 0.1 inch per day; or (3) any accelerating convergence between the roof and the floor. At the present time, the hereinabove listed criteria are used to determine whether preventative measures, such as roof bolting or cribbing, be installed to prevent subsequent roof fall.
At the present time, roof motion is measured using a dial gauge indicator between reference points anchored in the floor and the roof.
In a typical longwall mining operation, a longwall face could contain as many as 100 of these reference points, sometimes called "convergence stations" or "sag stations".
Positive convergence can be defined as the decrease in the distance between a reference point in the floor of a mine and a reference point attached to the roof. Such positive convergence can arise from either the roof sagging or the floor rising.
In order to measure "sag", a reference point is anchored approximately 8 to 15 feet above the mine roof in stable rock which is presumed not to move. The roof sag is then determined by subtraction of the floor motion relative to this point from the floor motion relative to the anchor.
It should be appreciated that a large number of stations required to monitor an entire mine requires the full-time effort of a number of mine personnel.
Automation of mine convergence is made difficult, and to date no present system is available, because of the stringent environmental constraints on a system designed for use in the mine. For example, in a longwall mining operation approximately half of the convergence stations will be in the return air shaft from the longwall. In these particular locations, substantial amounts of methane gas and coal dust are present. The Mine Safety Health Administration (MSHA) requires that all equipment located in such shafts be certified as "intrinsically safe". This requirement effectively bars any electrical sensing equipment from these locations.
In addition, the presence of high humidity, high dust levels, and mine activity including the movement of heavy machinery, dictates that any sensor or system for measuring and monitoring convergence be very durable. The sensors and system of the present invention are "intrinsically safe" and durable, thereby enabling the continuous monitoring of convergence in a great number of locations in an underground coal mining operation with a significant reduction in the manpower necessary to operate.